A passive optical network (PON) is one system for providing network access over “the last mile.” The PON is a point to multi-point network comprised of an optical line terminal (OLT) at the central office, an optical distribution network (ODN), and a plurality of optical network units (ONUs) at the customer premises. Time Division Multiplexing (TDM) PONs, such as Gigabit PONs (GPONs) and Ethernet PONs (EPONs), can serve potentially millions of users worldwide. In some PON systems, such as GPON systems, downstream data is broadcasted at about 2.5 Gigabits per second (Gbps) while upstream data is transmitted at about 1.25 Gbps. However, the bandwidth capability of the PON systems is expected to increase as the demands for services increase. To meet the increased demand in services, the logic devices in emerging PON systems, such as Next Generation Access (NGA), are being reconfigured to transport the data frames at higher bandwidths, for example at about ten Gbps, and to support a larger number of ONUs.
Optical network units (ONUs) utilizing such technology commonly share upstream bandwidth using time division multiple access (TDMA) schemes. More specifically, data from ONUs are sent to optical line terminals (OLTs) burst-by-burst. Burst data from different ONUs frequently contains analog signals of different amplitudes and phases.
Achieving burst-mode clock and data recovery in a short time period is desirable for compliance with the relevant standards. For example, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.984 standard specifies a 44 bit period recovery for GPON. Prior approaches to burst-mode clock and data recovery included phase locked loops (either analog or digital) and fast clock recovery circuitry. Because phase locked loops require thousands of bits to achieve suitable phase locking, conventional PON systems use fast clock recovery circuitry. Conventional fast clock recovery circuits generally employ one of two approaches: gated voltage controlled oscillators (gated VCOs) or over-sampling. Gated VCO systems are inexpensive but demonstrate relatively poor performance characteristics. Over-sampling systems demonstrate relatively better performance characteristics but are expensive and difficult to implement as data rate in PON upstream increases. Thus, a need exists for a relatively low-cost means for clock and data recovery for burst mode transmissions with relatively good performance characteristics.